Do you go short on one putt then long on the next?
The problem could be that you are not putting by feel. Great putters go by feel. Try this drill:
On the practice green, line up a putt. Just before you putt, close your eyes. Make your stroke. Before opening your eyes, try to guess if your ball ended up long, short, or in the hole. Try this drill from all different lengths as a routine part of your practice.